Buckles for straps are known in the art. In particular, buckles in the form of tri-bar sliders, rings and g-shaped hooks are commonly used with straps for garments such as fabric shoulder straps on women's brassieres. Such buckles may be attached to two ends of a garment strap and are slidable to adjust the fit of the garment on its wearer. Such buckles, however, may slide and move with respect to a strap made of a smooth material such as a smooth fabric, knitted elastic or ribbon, which results in loosening of the strap over the course of a day that the garment is worn. The present invention represents an improved buckle for straps that prevents loosening of the straps once the buckle has been moved into place.